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### Transmission of Electric Power

Transmission of Electric Power

Transmission of electric power is a process by which the electric power is transported over long distances for eventual use by consumers. Electric power is transported over long distances at high voltages, which minimizes the loss of electricity. It is sent from generating power plants to the end consumer by transmission lines. Electricity by nature is difficult to store, hence the supply is to equal the demand at any given instant.

The transmission system is the bulk power transfer system between the power generating station and the distribution centre from which power is carried to the customer delivery points. The transmission system includes step-up and step-down transformers at the generating and distribution stations respectively. The transmission system is normally part of the electric utility’s network. It can include sub-transmission stages to supply intermediate voltage levels. Sub-transmission stages are used to enable a more practical or economic transition between transmission and distribution systems

The basic electric terms used in transmission of electric power are (i) voltage, (ii) current, (iii) frequency, (iv) power, (v) energy, and (vi) corona loss. Voltage is the electrical ‘pressure’ measured in volts. For power systems, voltage is typically measured in thousands of volts or kilovolts (kV). Current is the movement of charge (electrons) through a conductor. It is measured in amperes (A). In alternating current (AC), the magnitude of current and voltage varies with time. Majority of the grids are AC grids. Alternating current frequency is the number of cycles per second in an AC sine wave. Frequency is the rate at which current changes direction per second. It is measured in hertz (Hz), which is an international unit of measure where 1 hertz is equal to 1 cycle per second. Normal AC frequency is either 50 Hz or 60 Hz. In direct current (DC), the magnitude of current and voltage is constant.  High voltage direct current (HVDC) grids are used for the transmission of power at very high voltage. Power is the rate at which electricity does work. It is measured in Watts or more typically kilowatts (kW) or megawatts (MW). Energy is the quantity of work which can be done by electricity. It is measured in Watt-hours or more typically kilowatt-hours (kWh) or megawatt-hours (MWh).  Corona loss is a major type of power loss in transmission lines. Essentially, corona loss is caused by the ionization of air molecules near the transmission line conductors. These coronas do not spark across lines, but rather carry current (hence the loss) in the air along the wire.

Power stations are connected with substations through transmission lines.  Transmission lines, when interconnected with power stations and with each other, form transmission network which is known as the ‘power grid’. The regional transmission networks are interconnected at the national level and this entire connected network forms the ‘national grid’. The transmission network comprises the land, conversion structures and conversion equipment at the generating stations and receiving stations, and all transmission lines and equipment whose main function is to increase, integrate, or tie together power supply sources to the load centres. A highly interconnected grid has higher reliability.

The large network of conductor between the power station and the consumers can be broadly divided into two parts namely (i) transmission system, and (ii) distribution system. Each part can further be sub-divided into two, primary transmission and secondary transmission, and primary distribution and secondary distribution. Fig 1 gives a typical single line diagram of a transmission network.

Fig 1 Typical single line diagram of a transmission network

Electrical grids are interconnected regional systems. An event in one location, affects other locations. To ensure reliable operation the system is to be in balance at all times. Planning is performed in collaborative forums, to ensure that the system operates reliably in accordance with technical standards. Interconnection frequency needs to be maintained close to 50 Hz / 60 Hz at all times for any instantaneous demand.

The power grid can be broken down in to four main components namely (i) generation, (ii) transmission, (iii) distribution, and (iv) load. Loads can be smaller than the cell phone hooked to its wall charger (say 1 watt) or as large as needed an industrial facility (in the tens of millions of watts).

Unlike highways, pipelines, and telecommunication lines, the flow of electricity on the AC grid cannot be easily routed or controlled. Power flow is dictated by laws of physics. Power flows through the path of least resistance. This is a critical difference in how the grid differs from other transportation mechanism.

Energy is normally transmitted within a grid with three phase alternating current (AC). Because of the involvement of large quantity of electric power and because of the properties of electricity, transmission involving long distances normally takes place at high voltage. The transmission network consists of the generating facilities, transmission lines, sub-transmission lines, distribution lines, and substations. Fig 2 shows typical power transmission network.

Fig 2 Typical power transmission network

Electricity is produced in generators at a generating station (power plant). The generator converts mechanical energy or solar energy to electrical energy by forcing electrical current to flow through an external circuit. Turbines create mechanical energy which are run by steam (produced in boilers utilizing fossil fuels, nuclear energy, or solar energy), falling water, or wind power. At the generating station electric power is produced by 3-phase alternators operating in parallel. The normal generation voltage is 11 kV. For economy in the transmission of electric power, the generation voltage (i.e., 11 kV) is stepped up to 132 kV (or more) at the generating station with the help of 3-phase step up transformers.

The transmission of electric power at high voltages has several advantages including the saving of conductor material and high transmission efficiency. It is advisable to use the highest possible voltage for transmission of electric power to save conductor material and have other advantages. But there is a limit to which this voltage can be increased. It is because increase in transmission voltage introduces insulation issues as well as the cost of switchgear and transformer equipment is increased. Hence, the choice of proper transmission voltage is essentially a question of economics. Normally, the primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV.

In case of primary transmission, the electric power at 132 kV or more is transmitted from the generating station by 3-phase, 3-wire overhead system to the outskirts of the city /steel plant. The primary transmission line is terminated at the receiving station which normally lies at the outskirts of the city /steel plant. At the receiving station, the voltage is reduced to 33 kV by step-down transformers. From this station, electric power is transmitted at 33 kV by 3-phase, 3-wire overhead system or underground cable system to different sub-stations located at the strategic points in the city / steel plant. This forms the secondary transmission.

Electricity lines and cables conductors are normally made of either aluminum or copper. The selection choice between the two is based on (i) electrical conductivity, (ii) weight, strength, and durability, (iii) cost, and (iv) flexibility in installation. Aluminum conductor can be either aluminum cable or can be aluminum conductor steel reinforced (ACSR). The advantages of aluminum conductor are (i) lower weight (lighter) than copper, and (ii) cheaper than copper. ACSR are primarily used for mid-voltage and high-voltage transmission lines. The advantages of copper conductor are (i) better conductivity, (ii) less insulation and armouring, and (iii) higher resistance to heat. Copper conductors are primarily used for low-voltage transmission lines.

The secondary transmission line terminates at the sub-station where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important road sides of the city / plant. This forms the primary distribution. It is to be noted that big consumers are normally supplied power at 11 kV for further handling with their own sub-stations.

In case of secondary distribution, the electric power from primary distribution line (11 kV) is delivered to distribution sub-stations. These sub-stations are located near the consumers’ areas and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution. The voltage between any two phases is 400 V and between any phase and neutral is 230 V.

The single-phase lighting load is connected between any one phase and neutral, whereas 3-phase, 400 V motor load is connected across 3-phase lines directly. It is worthwhile to mention here that secondary distribution system consists of feeders, distributors, and service mains. Feeders radiating from the distribution sub-station supply power to the distributors. No consumer is given direct connection from the feeders. Instead, the consumers are connected to the distributors through their service mains.

The transmission lines carry electricity over long distances, from the generating facility to areas of demand. The electricity in transmission lines is transported at high voltage. The level of voltage depends on the distance to which power is to be transmitted. It can vary from 11 kV to 400 kV or even higher. Transmission lines are normally attached to large lattice steel towers or tubular steel poles.

A transmission substation connects two or more transmission lines and contains high-voltage switches which allow lines to be connected or isolated for maintenance. The substation is also referred to as a switching station. The substation can have transformers to convert between two transmission voltages, or equipment such as phase angle regulators to control power flow between two adjacent power systems. A large transmission substation can need large land area with multiple voltage levels, and a large number of protection and control equipment such as capacitors, relays, switches, breakers, potential (voltage) transformers (PT) and current transformers (CT).

Sub-transmission lines carry electricity at voltages less than 200 kV. These lines are normally suspended on tall light-weight steel poles. They can also be placed underground.

A distribution substation reduces voltage from the high-voltage transmission system to a lower voltage suitable for the local distribution system of an area. It is uneconomical to directly connect electricity consumers to the high voltage transmission network, unless they use large quantities of energy. Distribution substations are normally located closer to the consumers.

From the distribution substation, electricity is transferred to distribution lines. These lines cover short distances, and are typically energized at low voltages e.g., 11 kV. Lower-voltage distribution lines carry electricity to neighborhoods on shorter steel poles or underground. Transformers located on distribution poles, or on a concrete pad on the ground, or underground further step down the voltage before it is ultimately delivered to the domestic or commercial consumers.

Electric power is normally transported to a substation near the consuming point which is either a populated area or an industrial complex. At the substation, the high voltage electric power is converted to lower voltages suitable for consumer use, and then transported to the end users through low voltage electric distribution lines.

The electric supply system can be broadly classified into (i) AC system or DC system, and (ii) overhead system or underground system. Now-a- days, 3-phase, 3-wire AC system is universally adopted for generation and transmission of electric power as an economical proposition. The underground system is more expensive than the overhead system. Hence overhead system is normally adopted for transmission and distribution of electric power.

According to type of construction, distribution system can be classified as (i) overhead system, and (ii) underground system. The overhead system is normally used for distribution as it is 5 times to 10 times cheaper than the equivalent underground system. Overhead lines are equipped with ground conductor (shield wire or earth wire). It is grounded conductor at the tower top to minimize the likelihood of direct lightning strikes to the phase conductors. Sometimes there are two ground conductors. Ground conductor can include optical fibers which are used for communication and control. Tab 1 shows difference between overhead and underground line system.

 Tab 1 Differences between overhead and underground line system Sl. No. Overhead system Underground system 1 Reliability is low Reliability is high 2 System is very cheap as no insulation coating is used over the conductors, i.e., the conductors used are bare conductors System is very costly, since a number of insulation layers has to be used on the conductor to provide sufficient insulation 3 Faults can be easily detected and maintenance is very simple Fault detection is complicated and maintenance is complex 4 The quantity of insulation needed is less since air provides necessary insulation High degree of insulation is needed 5 It can have interference with communication lines It does not have any interference with communication lines 6 There is liable hazards because of lightning discharges There is no hazards because of the lightning discharges 7 For same quantity of power, less conductor size is used It needs quite large size of conductor for same power 8 Low level of public safety High level of public safety 9 The system cannot be used near submarine crossings The system can be used near submarine crossings

In general, the underground system is used at places where overhead construction is impracticable or prohibited by the statutory regulations. Fig 3 shows overhead and underground transmission conductors.

Fig 3 Overhead and underground transmission conductors

Transmission efficiency and transmission losses – Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line. For a given quantity of power, a higher voltage reduces the current and hence the resistive losses in the conductor.

Transmission efficiency is improved by increasing the transmission voltage using a step-up transformer which has the effect of reducing the current in the conductors, whilst keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Ohms law, the losses are proportional to the square of the current, halving the current results in a four-fold decrease in transmission losses. Reduced current means lesser I-square R (square of the current ‘I’ multiplied by the conductor resistance ‘R’) loss in the system, less cross-sectional area of the electrical conductor cable means less capital involvement and decreased current causes improvement in voltage regulation of power transmission system and improved voltage regulation indicates quality of power. Because of these three reasons electrical power is mainly transmitted at high voltage level.

Hence, electric power to be efficiently transported to long distances need high voltages. This voltage can be 33 kV, 66 kV, 110 kV, 132 kV, 220 kV, 400 kV or even higher. The generator voltage of a power plant normally ranges from 11 kV to 25 kV. The generated electric power is first transported from the generator to a transformer at the power plant. The transformer increases the voltage to the voltage of the grid. The generator is then synchronized with the grid and the generated power is transmitted to the consumer end. At the consuming point end the transmission lines are connected to a substation. Here the transformers of substation change the voltage of the electric power from high voltage to a lower level. From substation electrical power of lower voltage is distributed to the consumers of the electrical power through distribution lines. The main components of an electric power transmission grid are described below.

Substation – Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Substation varies in size and configuration. Between the generating station and consuming point, electric power can flow through several substations at different voltage levels. A transmission substation connects two or more transmission lines. The simplest case is where all the transmission lines have the same voltage. In such case, substation contains high voltage switches which allow lines to be connected or isolated for fault clearance or maintenance.

A transmission station normally has transformers to convert between two transmission voltages, voltage control, power factor correction devices such as capacitors, reactors or static VAR (volts ampere reactive) compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small ‘switching station’ normally consists of a bus plus some circuit breakers. The large transmission substations normally are accommodated in a large area (several hectares) and have multiple voltage levels, several circuit breakers and a large quantity of protection and control equipment (voltage and current transformers, relays, and supervisory control and data acquisition, SCADA systems). Modern substations are installed as per international standards such as IEC Standard 61850.

Substations vary in size and configuration but can cover large areas. They are cleared of vegetation and typically surfaced with gravel. They are normally fenced, and are reached by a permanent access road. In general, substations include a variety of structures, conductors, fencing, lighting, and other features which give the substation an ‘industrial’ appearance. Fig 4 shows schematics of substation showing its main components.

Fig 4 Schematics of substation showing its main components

Components of substation are power transformers, auxiliary transformer, current transformer (CT), potential (PT), circuit breakers (CB), isolators, and bus bars. Power transformers are used for generation and transmission network for stepping-up the voltage at generating station and stepping-down the voltage at distribution substation. Auxiliary transformers supply power to auxiliary equipments at the substations.

The lines in substations carry currents in the order of thousands of amperes. The measuring instruments are designed for low value of currents. Current transformers are connected in lines to supply measuring instruments and protective relays. The lines in substations operate at high voltages. The measuring instruments are designed for low value of voltages. Potential transformers are connected in lines to supply measuring instruments and protective relays. These transformers make the low voltage instruments suitable for measurement of high voltages. For example, a 11kV / 110V potential transformer is connected to a power line which has the line voltage is 11 kV then the secondary voltage is 110 V which is suitable for the measuring instruments.

Circuit breakers are used for opening or closing a circuit under normal as well as abnormal (faulty) conditions. Different types of CBs which are normally used are oil circuit breaker, air-blast circuit breaker, vacuum circuit breaker, and SF6 circuit breaker. Isolators or Isolating switches are used in substations to isolate a part of the system for general maintenance. Isolator switches are operated only under no load condition. They are provided on each side of every circuit breaker. When number of lines operating at the same voltage levels needs to be connected electrically, bus-bars are used. Bus-bars are conductors made of copper or aluminum, with very low impedance and high current carrying capacity.

Bus-bars are the important components in a sub-station. There are several bus-bar arrangements which can be used in a sub-station. The choice of a particular arrangement depends upon different factors such as system voltage, position of sub-station, degree of reliability, and cost etc. Different types of bus-bar arrangements are (i) single bus bar system, (ii) single bus-bar with sectionalization, (iii) double bus-bar arrangements, (iv) sectionalized double bus-bar arrangement, (v) double main and auxiliary bus-bar arrangement, (vi) breaker and a half scheme / 1.5 breaker scheme, and (vii) ring bus-bar scheme

Single bus-bar system which consists of a single bus-bar and all the incoming and outgoing lines are connected to it. The chief advantages of this type of arrangement are low initial cost, less maintenance and simple operation. However, the principal disadvantage of single bus-bar system is that if repair is to be done on the bus-bar or a fault occurs on the bus, there is a complete interruption of the supply. This arrangement is not used for voltages higher than 33 kV. The indoor 11 kV sub-stations frequently use single bus-bar arrangement.

In single bus-bar system with sectionalization arrangement, the single bus-bar is divided into sections and load is equally distributed on all the sections. Any two sections of the busbar are connected by a circuit breaker and isolators. Two principal advantages of this arrangement are (i) if a fault occurs on any section of the bus, that section can be isolated without affecting the supply from other sections, and (ii) repairs and maintenance of any section of the busbar can be carried out by de-energizing that section only, eliminating the possibility of complete shut-down. This arrangement is used for voltages up to 33 kV.

Duplicate bus-bar system consists of two bus-bars, a ‘main’ bus-bar and a ‘spare’ bus-bar. Each bus-bar has the capacity to take up the entire sub-station load. The incoming and outgoing lines can be connected to either bus-bar with the help of a bus-bar coupler which consists of a circuit breaker and isolators. Ordinarily, the incoming and outgoing lines remain connected to the main bus-bar. However, in case of repair of main bus-bar or fault occurring on it, the continuity of supply to the circuit can be maintained by transferring it to the spare bus-bar. For voltages higher than 33 kV, duplicate bus-bar system is frequently used.

Transmission towers

Transmission towers are the most visible component of the power transmission system. They are used in high voltage AC and DC systems. A transmission tower is normally a tall steel structure. Its function is to keep the high-voltage conductors (power lines) separated from their surroundings and from each other. A wide variety of tower shapes, sizes, and designs exist which normally use an open lattice work or a tubular structure. Transmission towers are normally very tall with minimum height ranging from 10 metres (m) to 50 m and cross arms as much as 30 m wide. The height of the tower is the sum of the minimum permissible ground clearance, maximum sag, vertical spacing between conductors, and vertical clearance between earth wire. In addition to steel, other materials can be used, including concrete and wood. There are four major categories of transmission towers. They are suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions.

The transmission towers are to be designed to carry three (or multiples of three) conductors. The towers are normally made of steel lattices or trusses. The insulators are either glass or porcelain discs assembled on strings or on long rods whose lengths are dependent on the line voltage and environmental conditions. Typically, one or two ground wires, also called ‘guard’ wires, are placed on top to intercept lightning and harmlessly divert it to ground. Towers for high and extra high voltage are normally designed to carry two or more electric circuits.  Fig 5 shows typical shapes and heights of transmission towers.

Fig 5 Typical shapes and heights of transmission towers

Transmission lines – Transmission lines are also known as ‘tie lines’. They connect the individual substations with each other. Electric power is transmitted at high voltage (110 kV or above) to reduce the energy lost in long-distance transmission. Power is normally transmitted through overhead power lines. Underground power transmission has a considerably higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

Transmission lines normally use high voltage three-phase alternating current. High voltage direct current (HVDC) technology is used for greater efficiency in very long distances (typically several hundreds of kilometers).  A HVDC electric power transmission system is also called a power superhighway or an electrical superhighway. It uses direct current for electric power transmission, in contrast with the more common AC transmission systems. Majority of the HVDC links use voltages between 100 kV and 800 kV. However, a 1,100 kV link in China was completed in 2019 over a distance of 3,300 km with a power capacity of 12 GW (giga-watt). With this dimension, intercontinental connections become possible which can help to deal with the fluctuations of wind power and photovoltaics.

HVDC allows power transmission between AC transmission systems which are not synchronized. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows the transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing the exchange of power between previously incompatible networks.

HVDC links are also used to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures. Normally HVDC transmission needs converter station at both sending end and receiving end. The converter station are transformers and thyristor valves. At sending end thyristor valves act as rectifier to convert AC into DC which is transmitted over the line, whereas at the receiving end thyristor valves act as inverter to convert DC into AC which is utilized at receiving end. Each converter can function as rectifier or inverter. Hence, power can be transmitted in either direction.

Normally, several conductors are strung on a transmission tower for each electrical circuit. Conductors are constructed primarily of twisted metal conductors. High voltage overhead conductors are not covered by insulation. The conductor is normally ACSR conductor which is a specific type of high-capacity, high-strength stranded conductor.  The outer strands are made from hard drawn aluminum wire produced from not less than 99.5% pure electrolytic aluminum rods of EC grade and copper content not exceeding 0.04 %. High purity aluminum alloy is chosen for its excellent conductivity, low weight, and low cost. The central strands are of steel to provide the strength needed to support the weight without stretching the aluminum because of its ductility. This gives the conductor an overall high tensile strength. Copper is also used for overhead transmission but aluminum is lighter, yields only marginally reduced performance and costs much less.

Insulators – Transmission line insulators are devices used to contain, separate, or support electrical conductors on high voltage electricity supply networks. Transmission insulators come in various shapes and types, including individual or strings of disks, line posts or long rods. They are made of polymers, glass and porcelain, each with different densities, tensile strengths and performing properties in adverse conditions.  Types of insulators include (i) pin type, (ii) suspension type, (iii) strain insulator, and (iv) shackle insulator.

As the name suggests, the pin type insulator is secured to the cross arm on the pole. There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor. Pin type insulators are used for transmission and distribution of electric power at voltages up to 33 kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence uneconomical.

In case of suspension type insulators, the conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage, say 11 kV. The number of discs in series obviously depends upon the working voltage. For example, if the working voltage is 66 kV, then six discs in series are provided on the string.

Strain type insulators are used when there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators.

Shackle type insulators which are similar to strain type insulators, are used on sharp curves, end poles, and in section poles. However, unlike strain insulators, shackle insulators are designed to support lower voltages. These insulators are single, round porcelain parts which are mounted horizontally or vertically. In earlier days, the shackle insulators have been used as strain insulators.

Stay insulators are also called egg insulators. They are primarily used to prevent stay wires from becoming energized from accidentally broken live wires. Hence, they function to provide insulation between stay clamps and transmission poles. Stay insulators are mounted at a height of at least 3 meters from ground level.

Rights of way and access roads – The rights of way (ROW) for a transmission corridor includes land set aside for the transmission line and associated facilities, needed to facilitate maintenance, and to avoid risk of fires and other accidents. It provides a safety margin between the high voltage lines and surrounding structures and vegetation. Some vegetation clearing can be needed for safety and / or access reasons. A ROW normally consists of native vegetation or plants selected for favourable growth patterns (slow growth and low mature heights). However, in some cases, access roads constitute a portion of the ROW and provide more convenient access for repair and inspection vehicles. The width of a ROW varies depending on the voltage rating of the transmission line. Access roads to transmission line structures for both line construction and maintenance are normally needed, and can be paved or gravel.

Comparison of DC and AC transmission – The electric power can be transmitted either by means of DC or AC. Each system has its own merits and demerits. Now-a-days, electrical energy is almost exclusively generated, transmitted and distributed in the form of AC.

The advantages of high voltage DC transmission over high voltage AC transmission are (i) it needs only two conductors as compared to three for AC transmission, (ii) there is no inductance, capacitance, phase displacement and surge issues in the DC transmission, (iii) because of the absence of inductance, the voltage drop in a DC transmission line is less than the AC line for the same load and sending end voltage, hence, a DC transmission line has better voltage regulation, (iv) there is no skin effect in a DC system and hence the entire cross-section of the line conductor is utilized, (v) for the same working voltage, the potential stress on the insulation is less in case of DC system than that in AC system and hence, a DC line needs less insulation, (vi) a DC line has less corona loss and reduced interference with communication circuits, (vii) the high voltage DC transmission is free from the dielectric losses, particularly in the case of cables, and (viii) in DC transmission, there are no stability problems and synchronizing difficulties.

The disadvantages of DC transmission are (i) electric power cannot be generated at high DC voltage because of commutation issues, (ii) the DC voltage cannot be stepped up for transmission of power at high voltages, and (iii) the DC switches and circuit breakers have their own limitations.

The advantages of AC transmission are (i) the power can be generated at high voltages. (ii) the maintenance of AC sub-stations is easy and cheaper, (iii) the AC voltage can be stepped up or stepped down by transformers with ease and efficiency which permits to transmit power at high voltages and distribute it at safe potentials.

The disadvantages of AC transmission are (i) an AC line needs more copper than a DC line. (ii) the construction of AC transmission line is more complicated than a DC transmission line, (iii) because of the skin effect in the AC system, the effective resistance of the line is increased, and (iv) an AC line has capacitance, and hence, there is a continuous loss of power because of charging current even when the line is open.

There are several limitations of a transmission system. The thermal limitations include overheating of lines, transformers, and components, as well as line sag. The system stability can be angular disturbances on the system (switching, and contingencies etc.) which can cause the system to become unstable, or voltage disturbances since high demand / loading on transmission can cause voltages to become unstable and difficult to control.

Transmission systems have contingencies. Some capability left unused to handle failures. System limitations create congestion. All the aforementioned limitations are worsened by the lack of appropriate transmission. These limitations create congestion on the system which results in (i) uneconomic use of generation, (ii) re-dispatch means using less economic generators, (iii) reserve margins need to be higher to maintain reliability, (iv) potential for power consumption increases, and (v) need for ancillary services.